Device and a method for applying a coating on a workpiece by electrodeposition

ABSTRACT

The invention relates to a device and a method of applying a coating on a workpiece by electrodeposition. A vessel is provided that is suitable for filling with a bath of electrolyte, anode-forming conductor means being placed in the vessel and connected to a current generator, a cathode-forming workpiece mounted on the mandrel of a lathe, and guidance and movement means for guiding and moving the vessel relative to the lathe, the guidance and movement means enabling the workpiece to be immersed in full or in part in the bath of electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application under §1.53(b) of priorapplication Ser. No. 12/546,246, filed Aug. 24, 2009, entitled: A Deviceand a Method for Applying a Coating on a Workpiece by Electrodeposition,the entire disclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a device and to a method for applying acoating on a workpiece by electrodeposition.

BACKGROUND OF THE INVENTION

The usual methods of depositing a metal coating on a workpiece consistin immersing said workpiece in a vessel that contains a bath ofelectrolyte together with electrode panels. Those methods, also referredto as in situ electrodeposition methods, present drawbacks in terms ofduration and quality. Because of corner effects associated with theelectric field, the deposit builds up faster at the ends of theworkpiece. In order to obtain a coating that is uniform, it is thereforenecessary to perform a plurality of deposition operations in succession,and interrupt them with stages of machining in order to remove theirregularities progressively. The workpiece is inserted a first time inthe vessel containing the bath of electrolyte in order to receive afirst deposit, then it is withdrawn from the vessel and mounted on themandrel of a lathe so as to be machined. It is then introduced a secondtime in the bath of electrolyte to receive a second deposit, and thestages of deposition and of machining are thus repeated in alternationuntil a satisfactory coating is obtained. Methods using direct current(DC) baths generally require four to six passes, thereby giving rise toa significant loss of time and to large costs. One known solution forimproving the uniformity of the deposit consists in using an alternatingcurrent (AC) bath. That technique requires only a limited number ofpasses and enables a deposit to be obtained that is more uniform, but itdoes not avoid the need for the machining stage. In addition, it givesrise to problems associated with geometry and with keeping the chemistryof the bath constant.

Another major drawback of known electrodeposition methods is the need toregenerate the bath of electrolyte regularly. In the initial bath, theconcentration of ions available for electrolysis decreases as a resultof the cathode reaction of deposition on the workpiece. A commonly usedsolution for keeping the concentration of said ions constant is referredto as “blending” the bath and consists in periodically removing a volumefraction from the bath and replacing it with an equivalent fraction ofconcentrated new bath. That solution remains laborious. A solution thatenables the electrolytes to be regenerated continuously without externalintervention is known from French patent FR 2 821 627. That documentdescribes a method of electrodepositing nickel on a workpiece, themethod involving a vessel containing a conductor material fastened toone of the end faces of the vessel and forming an anode, with asufficient quantity of nickel beads for maintaining permanent contactwith said material. The cathode-forming workpiece that is to be coveredin nickel is situated under said vessel. Continuously regenerated by thenickel beads, the electrolyte comes by gravity into contact with theworkpiece and is recovered below so as to be reintroduced into thevessel. That method which takes place “outside” the vessel, neverthelessdoes not reduce the length of time required for the mounting/removaloperations prior to each stage of deposition or of machining, whenever aplurality of passes are needed.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention thus seeks to simplify the electrodepositionmethod, and to reduce its costs, by reducing the handling needed forobtaining a product that is uniform and of good quality, and also byreducing the duration of each pass.

More particularly, in a first aspect, the present invention provides anelectrodeposition device for performing electrodeposition on aworkpiece, the device comprising a vessel suitable for being filled witha bath of electrolyte, anode-forming conductor means placed in thevessel and connected to a current generator, a cathode-forming workpiecemounted on the mandrel of a lathe, and guidance and movement means forguiding and moving the vessel relative to the lathe between a firstposition enabling the workpiece to be immersed in full or in part in thebath of electrolyte, and a second position enabling the workpiece to bemachined.

In an advantageous disposition, the device includes at least a piece ofmetal for deposition that is suitable for remaining in permanent contactwith the conductor means during electrodeposition. Preferably, itcontains a set of metal beads made of the metal for deposition, placedin the vessel, and kept permanently in contact with the conductor meansunder the effect of gravity. For example, the metal for deposition isnickel.

According to another advantageous disposition, the vessel containing thebath of electrolyte includes forced circulation means for circulatingthe bath of electrolyte inside the vessel.

In particular, the conductor means may include a portion of a shape thatcorresponds to the outer longitudinal profile of the workpiece and thisportion is placed in the vessel so as to face the workpiece in the firstposition. In this way, the distance between the surface of thecathode-forming workpiece and the anode-forming conductor means is keptconstant and the deposition takes place in a manner that is moreuniform.

Advantageously, the vessel further includes accurate guidance meansenabling it to be moved vertically so as to conserve a constant distancebetween the cathode-forming workpiece and the conductor means duringelectrodeposition, thereby enabling the uniformity of the deposit to befurther optimized.

In particular, the device of the invention may be applied to acombustion chamber.

In a second aspect, the present invention provides an electrodepositionmethod of performing electrodeposition on a cathode-forming workpiece,the method consisting in:

a) mounting the workpiece on the mandrel of a lathe;

b) placing beneath the workpiece a vessel containing a bath ofelectrolyte together with conductor means forming an anode and placed inthe vessel connected to a current generator;

c) immersing all or part of the workpiece in said bath of electrolyte bymoving the vessel into a first position;

d) causing the workpiece to be turned by the lathe so that the entiresurface of the workpiece for coating is immersed at least once in thebath of electrolyte; and

e) moving the vessel containing the bath of electrolyte into a secondposition, so as to break contact between the workpiece and the bath ofelectrolyte.

In an advantageous disposition, after step e), the method includes astep f) consisting in machining the workpiece on the lathe. Inparticular, the series of steps b) to f) may be implemented a pluralityof times. It is thus possible to perform a plurality of successivepasses without removing the workpiece from the mandrel of the lathe.

In another advantageous disposition, a set of beads made of the metalfor deposition is put into permanent contact with the conductor meansunder the effect of gravity.

In order to enable the metal ions to disperse better in the bath ofelectrolyte, the bath may be put into forced circulation inside thevessel when the workpiece is immersed in full or in part therein.

The method may also include a step consisting in introducing waterand/or electrolyte into the bath of electrolyte in order to compensatefor losses due both to evaporation and to cathodic deposition.

Preferably, the vessel containing the bath of electrolyte is movedvertically during step d), such that a constant distance is conservedbetween the cathode-forming workpiece and the conductor means duringelectrodeposition.

Such dispositions enable the duration of the electrodeposition method tobe reduced significantly. Since the workpiece is mounted on the mandrelof a lathe and is immersed in full or in part in a bath of electrolytecontained in a movable vessel when located in the first position, it ispossible to perform the deposition and machining operations insuccession without any intermediate removal of the workpiece. After afirst deposition operation, the vessel is moved into the second positionso that the workpiece is no longer in contact with the bath ofelectrolyte. Machining can then be performed without moving or mountingthe workpiece beforehand on a lathe. After machining, the vessel isreplaced in the first position under the workpiece and a seconddeposition operation is performed. These steps are repeated until asatisfactory coating is obtained. By avoiding the mounting and removalstages concerning the workpiece and the lathe between the deposition andmachining operations, the method of the invention is faster than themethods known in the prior art.

Furthermore, great accuracy is achieved because, given that theworkpiece remains mounted on the lathe throughout the duration of themethod, there is no longer any need to re-set the reference thereofprior to each machining phase.

The particular shape of the conductor means, corresponding substantiallyto the shape of the workpiece that is to be coated, serves to avoidcorner effects and to improve the uniformity of the deposit. Betteruniformity is also achieved because of the fact that the precisionguidance means maintain a constant distance between the two electrodes,thereby avoiding variations in the intensity of the field lines.Finally, mounting the workpiece on the mandrel of a lathe enables theworkpiece to be rotated slowly during the electrodeposition method, suchthat its entire surface can be coated in uniform manner.

In addition to the above-described advantages, it is possible to furthersimplify the method by introducing a piece of the metal for depositioninto the vessel containing the bath of electrolyte, which piece is putinto permanent contact with the conductor means connected to the currentgenerator. In this way, the electrolyte of the bath is continuouslyregenerated without external intervention and implementation of themethod is thus facilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood and other advantages thereofappear better in the light of the following description given purely byway of example and made with reference to the accompanying drawings, inwhich:

FIG. 1 is a longitudinal section view of the device of the presentinvention in a first embodiment;

FIG. 2 is a cross-section view on line A-A showing the FIG. 1 device;and

FIG. 3 is a longitudinal section view of the device of the presentinvention in a second embodiment.

MORE DETAILED DESCRIPTION

The entire disclosure of U.S. application Ser. No. 12/546,246, filedAug. 24, 2009, is incorporated by reference herein.

FIG. 1 shows a first embodiment of the invention, applied to a workpiecethat is constituted by a cryogenic rocket engine combustion chamber(referred to below as the “chamber”). This type of chamber 1 is in theform of an annular copper ingot having channels 1 a referred to as“meridians” machined in the outer periphery thereof for the purpose ofallowing hydrogen to flow. These channels 1 a are filled with a specificwax, and the assembly is covered in a layer of nickel having a thicknessof 2 millimeters (mm) to 3 mm. The chamber 1 is of anaxially-symmetrical hourglass shape with a length of 600 centimeters(cm) to 700 cm. It comprises a first portion that is cylindrical with adiameter of about 50 cm and that is extended by a tapering secondportion that is frustoconical with a maximum diameter that correspondsto the diameter of the cylindrical first portion. The chamber 1 also hasa third portion that is frustoconical with a maximum diameter of about70 cm to 80 cm and that flares away from the second portion. Thedescription below relates to a device of the invention that enables saidlayer of nickel to be deposited by electrodeposition.

The chamber 1 is mounted centered on the mandrel 2 of a lathe 3 by meansof two supports 2 a in the form of bodies of revolution that are placedinside the chamber 1. In the vicinity of the chamber 1 there is placed avessel 4 containing a bath of electrolyte 5. The vessel is associatedwith means for guiding it and moving it (not shown) that make itpossible, after the chamber 1 has been mounted on the lathe 3, for thevessel to be moved reversibly under the chamber 1 so that at least afraction of the surface of the chamber 1 extending over the entirelength of the chamber along the axis of the mandrel 2 is immersed in theelectrolyte bath 5. In this way, rotating the mandrel 2 of the lathe 3,and thus rotating the axially-symmetrical chamber 1 mounted on themandrel, ensures that all of the outside surface of the chamber becomesimmersed. It is clear that under such circumstances, a workpiece havinga cross-section that is not circular (a workpiece that is not axiallysymmetrical) would require care to be taken to ensure that all of itszones for covering do indeed become immersed during rotation.

Conductor means 6 are placed in the vessel 4. They comprise a bottomportion 7 placed in the bottom of the vessel 4 and supporting a topportion 8 that is of a shape that corresponds at least approximately,but preferably accurately, to the outline of the chamber 1 along theaxis of the mandrel. This top portion 8 situated facing the chamber 1. Asubstantially constant distance is thus maintained between the chamber 1and the top portion 8 as a result of the similarity of their profiles,thus making it possible to avoid corner effects associated with theelectric field and thereby to ensure that deposition takes place moreuniformly. On the bottom portion 7 there are placed beads of nickel 9,at least some of which are in permanent contact with the bottom portion7 of the conductor means 6 under the effect of gravity. The conductormeans 6 are connected to the positive terminal of a current generator10, thus forming an anode, while the combustion chamber 1 is connectedto the negative terminal of the current generator, thereby forming thecathode. Under the effect of the potential difference exerted betweenthe two electrodes, the nickel beads in contact with the anode becomepolarized and release Ni²⁺ nickel ions that are suitable for picking upelectrons present on the cathode-workpiece (chamber 1), thereby creatinga deposit of solid nickel. Because of the beads of nickel, theconcentration of ions in its bath of electrolyte remains constant, andunlike known methods, there is no need to continuously renew theelectrolyte contained in the vessel.

The distance between the anode and the cathode decreases as thethickness of the deposit on the workpiece increases. In order toconserve a constant distance between the electrodes and to avoidvariations in the intensity of the field lines, precision guidance means(not shown) are preferably provided that enable the vessel to be movedaway vertically during the electrodeposition method.

A system is provided for circulating the bath of electrolyte in thevessel. By way of example, this system may be made up of two pipes 11and 12, each connected to the bottom portion and to the top portion of adifferent side face of the vessel, together with a pump 13. The bath ofelectrolyte is thus subjected to forced circulation between the bottomof the vessel where the beads of nickel are located and the top of thevessel where the immersed portion of the chamber is located. Thiscirculation serves to stir the electrolyte and to disperse the Ni²⁺nickel ions towards the workpiece that is to be covered.

A water tank 15 is also connected to the vessel 4 that contains the bathof electrolyte 5. This connection may be provided by a rigid duct, withthe tank 15 then being constrained to move vertically with the vessel 4.Alternatively, the pipe may be flexible and the tank may remainstationary during the various steps of the method. Such means forfeeding water to the vessel 4 serve to compensate for losses due inparticular to evaporation, and to maintain a constant volume ofelectrolyte in the vessel 4.

FIG. 2 is a cross-section view of the device of the invention. Thevessel containing the bath of electrolyte is in the form of a half-shellthat enables it to match the generally cylindrical and elongate shape ofthe chamber. Such a shape enables the volume of electrolyte needed to bediminished. For example, the vessel containing the bath of electrolytemay be semicylindrical in shape.

Further, if necessary, a gasket can be located between the mandrel 2 andthe edge of the vessel 4 so as to provide sealing and to avoid anyelectrolyte flowing out from the vessel via the ends.

FIG. 3 shows a second embodiment of the present invention. Elements thatare common with the first embodiment retain their reference numerals inthe description below.

A bracket 17 is fastened to one of the end walls of the vessel 4 (shownon the left in FIG. 3). It comprises a first portion 18 forming thebottom of the bracket, which portion is extended by a second portion 19of shape similar to that of the chamber 1, and a third portion 20connected to one or the other or both of the first and second portions.The bracket 17 forms an anode connected to the positive pole of acurrent generator. As in the above-described embodiment, the chamber 1forming the cathode is connected to the negative terminal of saidgenerator. The nickel beads 9 are retained in the vessel, in a housingdefined by the end wall and the fraction of the bracket 17 thatcomprises the portions 20 and 18. In this way, the beads 9 may be keptin permanent contact with the first portion 18 of the bracket 17.

The method of using the device of the invention is described below ingreater detail.

Initially, the combustion chamber 1 is mounted on the mandrel 2 of thelathe 3 with the help of the supports 2 a. It is preferably axiallysymmetrical and centered on the axis of the mandrel 2. The vessel 4 isfilled with the bath 5 of electrolyte and the anode-forming conductormeans 6 are put into contact with the nickel beads 9 and connected tothe positive terminal of the current generator 10. By means of suitableguidance and movement means, the vessel 4 is placed in the firstposition such that at least a portion of the chamber 1 is immersed inthe bath of electrolyte. The workpiece is then set into slow rotation bythe lathe. The generator produces DC, thereby polarizing the nickelbeads via the conductor means 6. Electricity is then conveyed to thecathode (chamber 1) by the Ni²⁺ ions which, on capturing electrons,enable nickel to be depositing on the surface of the chamber. Because ofthe particular shape of the top portion 8 of the conductor means 6,deposition takes place more uniformly than in prior art methods.Nevertheless, after each deposition operation it remains necessary tomachine the chamber in order to eliminate irregularities. The vessel isthus subsequently withdrawn and placed in the second position by itsguidance and movement means, and the workpiece is machined immediatelyon the lathe without being removed therefrom. Thereafter the vessel isreplaced in the first position and a second deposit is made. Assuccessive deposition operations take place, the accurate guidance meansmove the vessel and thus the anode in such a manner as to ensure thatthe distance between the anode and the cathode remains constant at alltimes.

Although the invention is described in the context of depositing nickelon the outside face of the workpiece, the method can be applied in thesame manner to the inside surface of the workpiece. Under suchcircumstances, conductor means are used that have a portion of shapethat corresponds to the longitudinal profile of the inside surface ofthe workpiece, said portion being placed inside the workpiece in thefirst position so as to face the portion of the workpiece that is to becovered.

In the context of the present invention, it is also possible to envisagean embodiment without metal beads, with a portion of the chamber 1 beingimmersed in the vessel 4 that is filled with a bath 5 of electrolytethat contains Ni²⁺ nickel ions and that is provided with electrodepanels. Such an embodiment is more constricting since, unlike theembodiment described above, it requires the electrolyte of the bath inwhich the chamber is immersed to be recharged regularly so as tomaintain a constant concentration of ions.

What is claimed is:
 1. An electrodeposition device for performingelectrodeposition on a workpiece, the device comprising a vesselsuitable for being filled with a bath of electrolyte, anode-formingconductor means placed in the vessel and connected to a currentgenerator, and a cathode-forming workpiece, wherein the device furthercomprises a lathe, the workpiece being mounted on the mandrel of saidlathe, and guidance and movement means for guiding and moving the vesselrelative to the lathe between a first position enabling the workpiece tobe immersed in full or in part in the bath of electrolyte, and a secondposition enabling the workpiece to be machined.
 2. An electrodepositiondevice according to claim 1, including a set of beads made of the metalfor deposition, the beads being placed in the vessel and being suitablefor remaining in permanent contact with the conductor means under theeffect of gravity while electrodeposition is taking place.
 3. Anelectrodeposition device according to claim 2, wherein the metal beadsare made of nickel.
 4. An electrodeposition device for performingelectrodeposition on a workpiece according to claim 1, wherein thevessel containing the bath of electrolyte includes forced circulationmeans for circulating the bath of electrolyte inside the vessel.
 5. Anelectrodeposition device according to claim 1, wherein the conductormeans include a portion of a shape that corresponds to the outerlongitudinal profile of the workpiece and which is placed in the vesselso as to face the workpiece in the first position.
 6. Anelectrodeposition device according to claim 1, wherein the vesselfurther includes accurate guidance means enabling it to be movedvertically so as to conserve a constant distance between thecathode-forming workpiece and the conductor means duringelectrodeposition.
 7. An electrodeposition device according to claim 1,wherein the vessel presents a half-shell shape.
 8. An electrodepositiondevice according to claim 1, wherein the volume of the bath ofelectrolyte is maintained constant by means for supplying water and/orelectrolyte.
 9. An electrodeposition device according to claim 1,wherein the workpiece is axially symmetrical.
 10. An electrodepositiondevice according to claim 1, wherein the workpiece is a combustionchamber.